It’s been eleven months and 76 blog posts, but Pembroke at last has an operational turntable! It has been quite a journey, to say the least. There is a step up onto the turntable on the side that had the great MDF debacle; I’m hoping it will not need shimming, but I will reserve judgement until I’ve driven over it many more times.
Before I go and reflect on what I learned, let’s recall why I went down this particular path.
- I was unable to find a commercial 50′ turntable. If I had found one, I would have used it, as I have no information to say that it would have been inaccurate.
- I have no machine tools, save a drill press.
- I believe that things on the railroad that were manual should be manual on the model. Thus, this needed a manual drive.
- Being Proto:87, I wanted an accurate alignment mechanism.
So, what did I learn along the road? What would I do differently if I were starting over?
Perhaps the most important thing I learned is to watch out for tolerances compounding on one another, “tolerance stack,” and to watch out for the poor tolerances that screws (especially wood screws) maintain. The main thing I would do differently if I were doing it again is to get someone to turn the pit out of a material that is dimensionally stable.
That’s only the start of course. It’s been a campaign rather than a battle, and it would have been a dead loss if I’d learned nothing along the way. Here, then, are some of the more important lessons:
- A 50’3″ turntable requires a pit that is larger than 50’3″ in diameter! I spent a lot of time sanding back the pit wall to clear the turntable.
- Measure from the centre, not from the pit wall, especially if the pit wall was sanded to shape.
- Small diameter shafts are not much good for applying torque, of which the lazy susan base required a surprising amount.
- Do not trust wood screws for alignment. Actually, I am still learning this one as there is a misalignment in the approach for the turntable. I’m going to leave it for now, and live with the derailments, in case I find myself building the roundhouse before the PNR convention this spring. If I do, I will fix the alignment after the convention.
- Ensure the tracks line up with the centre of the turntable as installed, not the centre as planned.
- Do not trust machine screws for alignment either. Use a more positive socket to align parts that need to stay aligned after being assembled and disassembled many times.
- Remember that small errors at the centre of a circle are amplified at the edge.
- You should measure the overall width of your locomotives before locking in the locations of any parts of the turntable that need to clear the equipment. Apparently I am not the only person to make this mistake, so watch out!
- I would not make the pit out of MDF; in fact I don’t think I will ever use MDF again, for anything! While this material was easy to shape with my router, water from the scenery process found its way past the primer and into the MDF, making it expand disastrously.
There are still a couple of things that I’m not 100% satisfied with. As I said, there is a little step onto the north end of the turntable due to the MDF problems. Also, the flex shaft that drives the turntable is a little too flexible, and may want replacing with a cardan shaft. Finally, there is a misalignment in the approach rails, which I will correct once I’ve decided what I’m doing about the roundhouse.